The combination of a d.c. signal with an a.c. signal is widely known. The d.c. voltage signal is used as an auxiliary means to detect closure of the subscriber's loop when the called subscriber lifts the handset from its cradle.
In contemporary telephone exchanges a sinusoidal ringing voltage is superimposed on the DC-battery voltage, (FIG. 1 illustrates the voltage forms Va and Vb of the a- and the b-wire of the subscriber's line during ringing at a battery voltage of 48 volts and a ringing voltage of 80 volts r.m.s.=113 volt peak-peak value). When the ringing voltage is controlled by an electronic circuit, the magnitude of the required supply voltage for the circuit poses a problem. In the case of FIG. 1 the required supply voltage, allowing for a safety margin, will have to be, for example, 170 volts.